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Mar 5, 2013 - ... L. Chaudhari. 3 Biotech 2017 7 (1), ..... Bharat Baruah. RSC Advances .... International Journal of Hydrogen Energy 2015 40 (14), 49...
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Facile Synthesis of Silver Nanoparticles Stabilized by Cationic Polynorbornenes and Their Catalytic Activity in 4‑Nitrophenol Reduction Bharat Baruah,* Gregory J. Gabriel,* Michelle J. Akbashev, and Matthew E. Booher Department of Chemistry and Biochemistry, Kennesaw State University, Kennesaw, Georgia 30144-5591, United States S Supporting Information *

ABSTRACT: We report the facile one-pot single-phase syntheses of silver nanoparticles stabilized by norbornene type cationic polymers. Silver nanoparticles (AgNPs) stabilized by polyguanidino oxanorbornenes (PG) at 5 and 25 kDa and polyamino oxanorbornenes (PA) at 3 and 15 kDa have been synthesized by the reduction of silver ions with NaBH4 in aqueous solutions at ambient temperature. The four different silver nanoparticles have been characterized by UV−vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and transmission electron microscopy (TEM) for their particle size distributions. Interestingly, PG stabilizes the silver nanoparticles better than PA as evident from our spectroscopic data. Furthermore, the AgNP-PG-5K (5K = 5 kDa) was found to serve as an effective catalyst for the reduction of 4-nitrophenol to 4aminophenol in the presence of NaBH4. The reduction has a pseudo-first-order rate constant of 5.50 × 10−3 s−1 and an activity parameter of 1375 s−1 g−1, which is significantly higher than other systems reported in the literature. acrylamide13 are also popular stabilizing agents. For example, Song et al.29 used cationic polymer nanofibers to embed silver nanoparticles to reinforce the antimicrobial activity of the embedded silver nanoparticles. Tan and co-workers reported12 the use of branched polyethylenimine (BPEI) and 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) together to reduce silver nitrate under UV irradiation for the synthesis of silver nanoparticles. The resultant silver nanoparticles are positively charged as the BPEI amino groups are protonated in the process. In this report, the ability of two chemically different polynorbornenes to stabilize AgNPs was studied. The two cationic polymers, at two different molecular weights each, shown in Figure 1, were made from oxanorbornene monomers via ring-opening metathesis polymerization (ROMP) following reports by Tew and co-workers.30,31 Polyguanidino oxanorbornene (PG) has been shown previously to have a remarkable combination of potent antimicrobial activity and low membrane-disruptive properties. Its amino counterpart, polyamino oxanorbornene (PA), interestingly does not possess these properties.30 Additionally, the cell-permeating activities of

1. INTRODUCTION In recent years, silver nanoparticles (AgNPs) have been gaining significant research interest due to their unique shape and sizedependent optical,1 antimicrobial,2 and catalytic properties.3 Nanostructures of silver such as monodispersed nanoparticles,4 nanoprisms,5 nanocubes,6 nanowires7 and nanodisks8 have potential applications in optics, catalysis, and SERS detection.9 The synthesis of silver nanoparticles involves three broadly defined routes. Silver nanoparticles can form in the presence of strong reducing agents, such as sodium borohydride,10 hydrazine,11 and tetrabutylammonium borohydride.11 A second route can involve the UV irradiation,12 γ-irradiation,13 ultrasound irradiation,14 or microwave15 heating of silver ions in solution in the presence of capping agents. A third route involves prolonged heating of silver ions in solution in the presence of weak reducing agents, such as sodium citrate,16 glucose,17 dimethylformamide,18 potassium bitartrate,19 ascorbic acid,20 and polyols.21 A wide variety of capping agents have been used to control size, shape, stability, and solubility of silver nanostructures. Organic thiol compounds22 are the most commonly used capping agents to stabilize colloidal silver. Long-chain amine,23 aniline,16 surfactant,24 carboxylates,4 and starch25 have been also used to stabilize silver nanostructures. In addition, polymers such as poly(vinylpyrrolidone),21 poly(amidoamine),26 polyacrylate,27 polyacrylonitrile,28 and poly© 2013 American Chemical Society

Received: December 20, 2012 Revised: February 27, 2013 Published: March 5, 2013 4225

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on the Boc-protected polymers with a Polymer Lab LC1120 pump equipped with a differential refractometer detector. The mobile phase used was THF with a flow rate of 1.0 mL/min at a temperature of 35 °C. Separations were performed with two Polymer Lab Resipore columns, and molecular weights were calibrated versus narrow molecular weight polystyrene standards. The reported weights of PG-5K, PG-25K, PA-3K, and PA-15K reflect the molecular weights of the Boc-protected polymers, and these polymers had an average degree of polymerization of 10, 54, 9, and 48, respectively. The GPCderived PDI of the polymers as defined by Mw/Mn were all between 1.05 and 1.15, and proton NMR of the crude reaction mixtures indicated complete incorporation of the monomer into the polymer. 2.3. Synthesis of PG-5K Stabilized Silver Nanoparticles (AgNP-PG-5K). The PG-5K-stabilized AgNPs were synthesized as follows: 0.2 mL of AgNO3 (100 mM) and 0.7 mL of PG-5K (2.5 mg/ mL) solutions were thoroughly mixed in a 20 mL scintillation vial containing 8.95 mL of vigorously stirred water. 0.15 mL of fresh NaBH4 solution in water (10 mM) was then slowly added into the mixed solution. The mixed solution immediately turned yellow upon the addition of NaBH4, indicating the rapid formation of AgNP-PG5K. The resulting AgNP-PG-5K solutions were left to stand overnight at room temperature to allow the residual NaBH4 to decompose before any characterization took place. Polymer bound AgNPs were centrifuged in 1.5 mL Eppendorf tubes at 12 000 rpm for 20 min to remove unbound polymer and excess salts. After completion the supernatant was discarded, and the AgNP-PG-5K particles were redispersed in 1.0 mL of DI water. 2.4. Synthesis of PG-25K Stabilized Silver Nanoparticles (AgNP-PG-25K). The PG-25K-stabilized AgNPs were synthesized as follows: 0.2 mL of AgNO3 (100 mM) and 0.4 mL of PG-25K (2.5 mg/ mL) solutions were thoroughly mixed in a 20 mL scintillation vial containing 9.24 mL of vigorously stirred water. 0.16 mL of fresh NaBH4 solution in water (10 mM) was then slowly added into the mixed solution. The mixed solution immediately turned yellow upon the addition of NaBH4, indicating the rapid formation of AgNP-PG25K. The resulting AgNP-PG-25K solutions were left to stand overnight at room temperature to allow the residual NaBH4 to decompose before any characterization took place. Polymer bound AgNPs were centrifuged in 1.5 mL Eppendorf tubes at 12 000 rpm for 20 min to remove unbound polymer and excess salts. After completion the supernatant was discarded, and the AgNP-PG-25K particles were redispersed in 1.0 mL of DI water. 2.5. Synthesis of PA-3K and PA-15K Stabilized Silver Nanoparticles (AgNP-PA-3K and AgNP-PA-15K). The PA-3Kand PA-15K-stabilized AgNPs were synthesized following exactly the same procedures as described above for AgNP-PG-5K and AgNP-PG25K, respectively. 0.7 mL of PA-3K (2.5 mg/mL) and 0.4 mL of PA15K (2.5 mg/mL) were used instead of 0.7 mL of PG-5K (2.5 mg/ mL) and 0.4 mL of PG-25K (2.5 mg/mL), respectively. 2.6. UV−Vis and Fourier Transform Infrared Spectroscopic Studies of Nanoparticles. The absorption spectrum was recorded using a Cary 4000 UV−vis spectrophotometer. FTIR spectroscopy was performed using a PerkinElmer FTIR Spectra 100 spectrometer fitted with a diamond ATR. The nanoparticles samples were first dried on the ATR crystal, and spectra were acquired at 400−4000 cm−1 wavenumbers with a 4 cm−1 resolution. The spectra were further analyzed by Origin 7.0 software. 2.7. Size Measurement with Dynamic Light Scattering. AgNPs were characterized in terms of sizes using dynamic light scattering with a commercial Zetasizer (Malvern Zetasizer Nano ZS, Malvern Instruments). For size measurements the instrument was calibrated using a 60 nm polystyrene standard. We loaded samples into disposable cells, and particle size measurements were done in duplicate. 2.8. Electron Microscopy. Transmission electron microscopy (TEM) images were collected with a Hitachi H-7500 transmission electron microscope (Hitachi High Technologies American, Pleasanton, CA) equipped with a Gatan BioScan CCD camera (Gatan Inc., Pleasanton, CA) operating at an acceleration voltage of 75 kV. Nanoparticle samples were prepared in DI water and were centrifuged

Figure 1. Structures of polyguanidino oxanorbornene (PG), polyamino oxanorbornene (PA), and a cartoon of the AgNP stabilized by PG or PA. Counteranion = trifluoroacetate. Two different molecular weights were targeted for each polymer, and the reported weights are of the Boc-protected polymers. For PG, made at 5 and 25 kDa, n = ∼10 and ∼54 and for PA, made at 3 and 15 kDa, n = ∼9 and ∼48.

polyarginines,32,33 synthetic guanidinium-containing polymers,34,35 and, in particular, the PG reported here36 have recently led to the development of several other polynorbornene derivatives that efficiently act as protein transduction mimics.37 While several groups have fabricated and evaluated the antimicrobial properties of materials consisting of silver nanoparticles and cationic polymers such as polyvinylpyridines38 and polyethylenimine,39 there are no analogous studies in the literature using membrane-penetrating norbornene-type cationic polymers as stabilizing agents. Here we report the facile synthesis of silver nanoparticles stabilized by PA and PG as tracked with UV spectroscopy, IR spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The size and the higher-order organization of the nanoparticles appear to be dependent on the type of cationic moiety present and the length of the polymer. Importantly, the silver nanoparticles synthesized in this report are also effective in catalyzing the reduction40,41 of 4-nitrophenol to 4-aminophenol in the presence of excess NaBH4 following pseudo-firstorder kinetics. This reduction is unfeasible in the presence of the strong reducing agent, NaBH4 without the nanoparticle catalyst. Since polyguanidino oxanorbornene (PG) is watersoluble and biocompatible,30,37 PG-protected nanoparticles may be comfortably integrated into systems related to drug delivery, biomedical, pharmaceutical, and biosensor applications. Exploitation of such polymer-bound nanoparticles in the area of biomolecular and catalytic applications has tremendous potential in the development of nanotechnologies related to medicine.

2. EXPERIMENTAL SECTION 2.1. Materials. Solvents and chemicals for the monomer synthesis were purchased from Fisher Scientific. For the polymer synthesis the second-generation Grubbs catalyst, (tricyclohexylphosphine) (1,3dimesitylimidazolidine-2-ylidine)benzylideneruthenium dichloride, was purchased from Aldrich and converted to a third-generation catalyst by reaction with 3-bromopyridine, from Aldrich, according to the literature. Polymerizations were performed in dry CH2Cl2 purchased sealed under nitrogen with molecular sieves from Acros. Silver nitrate, AgNO3 (99%), was purchased from Fisher Scientific. NaBH4 (98.5%) and 4-nitrophenol (98%) were purchased from Sigma-Aldrich. 2.2. Synthesis and Characterization of Polymers. The polyguanidino oxanorbornene samples at molecular weights of 5 and 25 kDa and the polyamino oxanorbornene samples at molecular weights of 3 and 15 kDa were synthesized according to the literature.30,31 Gel permeation chromatography (GPC) was performed 4226

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Figure 2. (A) UV−vis spectra of (a) 10 mM AgNO3 (black line), (b) aqueous PG-5K (red line), (c) aqueous PG-25K (green line), (d) AgNP-PG5K (blue line), and (e) AgNP-PG-25K (orange line). (B) UV−vis spectra of (a) 10 mM AgNO3 (black line), (b) aqueous PA-3K (red line), (c) aqueous PA-15K (green line), (d) AgNP-PA-3K (blue line), and (e) AgNP-PA-15K (orange line). Corresponding photographs of these solutions are shown in the inset.

Figure 3. (A) UV−vis spectra of (a) AgNP-PG-5K (blue line), (b) AgNP-PG-25K (orange line), a month old solution of (c) AgNP-PG-5K-1m (red line), and a month old solution of (d) AgNP-PG-25K-1m (green line). (B) UV−vis spectra of (a) AgNP-PA-3K (blue line), (b) AgNP-PA-15K (orange line), a month old solution of (c) AgNP-PA-3K-1m (red line), and a month old solution of (d) AgNP-PA-15K-1m (green line). Photographs of month old solutions are shown in the inset. and redispersed in DI water. These solutions were filtered through 200 nm syringe filters before applying on grids for TEM measurements. Samples were prepared by spreading a 5.0 μL of sample on an ultrathin 300 mesh Formvar/carbon-coated copper grid and dried in air. 2.9. Catalytic Study. The catalytic reduction reaction of 4nitrophenol was performed in aqueous solution in a standard quartz cell with a 1 cm path length. The reaction procedure was as follows: 0.12 mL of 0.1 M NaBH4 was mixed with 24 μL of 5.0 mM 4nitrophenol in a 1.5 mL Eppendorf tube, and the volume was adjusted to 1.0 mL with deionized water (DI). The overall concentrations of 4nitrophenol and NaBH4 were 0.12 and 12 mM, respectively. That lead to a color change from light yellow to yellow-green. The solution was transferred to a standard quartz cell (0.7 mL) for UV−vis measurements. For those samples with silver nanoparticles, 24 μL of 5.0 mM 4-nitrophenol and 25 μL of AgNP-PG-5K were mixed in a 1.5 mL Eppendorf tube, and then 0.12 mL of 0.1 M NaBH4 was added before adjusting the volume to 1.0 mL. Immediately after that, the solution was transferred to a standard quartz cell, and the UV−vis absorption spectra were recorded with a time interval of 80 s in a scanning range of 200−700 nm at ambient temperature (25 ± 2 °C).

3.1. Characterization of AgNPs by UV−vis Spectroscopy. UV−vis spectra of the AgNPs are presented in Figure 2A,B, and it is evident that metallic AgNPs were formed. The absorbance maximum observed at 392−420 nm is characteristic of silver surface plasmon resonance (SPR).42 Figure 2 shows the UV−vis spectra of the silver nanoparticles prepared with different polymers at the same AgNO3 concentrations. For comparison, the UV−vis spectra of AgNO3 and the polymer alone are presented along with their nanoparticles solutions. Figure 2A shows the UV−vis spectra of 10 mM AgNO3 (curve a), aqueous PG-5K (curve b), aqueous PG-25K (curve c), colloidal AgNP-PG-5K (curve d), and colloidal AgNP-PG-25K (curve e). In Figure 2A (curve a), there is only one signal at 301 nm characteristic of silver ion (Ag+) in aqueous solution; in curve b, the signal at 250 nm is characteristic of PG-5K in aqueous solution, and in curve c, the signal at 250 nm is characteristic of PG-25K. For curves d and e there is only a dominant absorption maximum at 399 nm (absorbance 0.54) and 400 nm (absorbance 0.46), respectively, characteristic of colloidal AgNPs. This is indicative of AgNP-PG-5K (curve d) and AgNP-PG-25K (curve e) nanoparticles being formed. The dispersity of AgNPs was evaluated by comparing the full width at half-maximum (fwhm)43 from the UV−vis spectra. The fwhm of colloidal AgNP-PG-5K and AgNP-PG-25K are 65 ± 1 and 80 ± 2 nm, respectively. This indicates that AgNP-PG-5K was relatively monodispersed and AgNP-PG-25K was relatively polydispersed.43 Figure 2B show data for studies with the PA samples. Curve a is for silver ion (Ag+) in aqueous solution,

3. RESULTS AND DISCUSSION Four different polymer-stabilized silver nanoparticles (AgNPs), AgNP-PG-5K, AgNP-PG-25K, AgNP-PA-3K, and AgNP-PA15K, were simply synthesized by reducing silver ion in aqueous solutions of the polymer as discussed in the Experimental Section. The physicochemical properties of the samples such as stability, size, morphology, and chemical activty were characterized. 4227

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Figure 4. (A) Hydrodynamic diameter of (i) AgNP-PG-5K (black line), (ii) AgNP-PG-25K (red line), (iii) AgNP-PA-3K (green line), and (iv) AgNP-PA-15K (blue line). (B) TEM images of (i) AgNP-PG-5K, (ii) AgNP-PG-25K, (iii) AgNP-PA-3K, and (iv) AgNP-PA-15K conjugates. Images were taken with a Hitachi H-7500 transmission electron microscope at an accelerating voltage of 75 kV. Samples were prepared by spreading 5 μL of aqueous sample on an ultrathin 300 mesh Formvar/carbon film on copper grid and dried in air.

c) after a month. In addition, the fwhm of colloidal AgNP-PA3K changes from 85 ± 2 to 73 ± 1 nm after a month. The decrease in absorbance is 0.14 units, and the shift in absorption position is 10 nm. These larger shifts indicate that surface silver atoms are oxidized significantly over the course of 1 month.46,47 However, AgNP-PA-3K essentially remains relatively polydispersed. The absorption maximum of AgNP-PA-15K (curve b) at 393 nm (absorbance 0.07) changes to 403 nm (absorbance 0.034) (curve d), and the shoulder at 485 nm disappears after a month. In addition, the fwhm of colloidal AgNP-PA-15K changes from 130 ± 3 to 74 ± 1 nm after a month. This change indicates that in the course of a month the large aggregates were dispersed to relatively larger AgNPs, and some surface silver atoms are also lost as Ag+ ions due to oxidation.46,47 It is worthwhile to mention here that we do not observe any absorption maximum for the dissociated silver ions (Ag+) in Figure 3 as they are under the detection limit. In Figure 2 it was shown that the absorption maximum of 10 mM silver ion has a absorbance of 0.06. During the synthesis of AgNPs we have used overall 10 mM of AgNO3, and the solution was allowed to stand overnight to allow all the Ag+ ions to reduce to elemental silver atoms. In addition, we have performed centrifugal purification to get rid of unreduced Ag+ ions. Hence, due to the oxidation of the AgNPs after 1 month, the solutions are not producing enough silver ion to be detectable by UV−vis spectroscopy. 3.2. Dynamic Light Scattering Size Data and TEM Images. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were employed to analyze the size and morphology of AgNPs. Dynamic light scattering (DLS) is the most versatile and useful set of techniques for measuring in situ the sizes, size distributions, and (in some cases) the shapes of nanoparticles in liquids.48 As shown in Figure 4A, the hydrodynamic diameter of AgNP-PG-5K was equal to 18 ± 1 nm (polydispersity index, PDI 0.090) (black line). The polydispersity index is dimensionless and scaled such that values smaller than 0.05 are rarely seen other than with highly monodisperse samples. A polydispersity index of 0.1 is suggested as a suitable limit of monodispersity and values greater than 0.7 indicate that the sample has a very broad size distribution.49 The corresponding TEM image in Figure 4B (i) supports this size distribution for AgNP-PG-5K. In Figure 4A three populations of the AgNP-PG-25K (red line) were found to be located in 0.6 nm (at 2% intensity), 2.0 nm (at 23%

curve b is for PA-3K in aqueous solution with a characteristic signal at 250 nm, and curve c is for PA-1K in aqueous solution with a characteristic signal at 248 nm. For curve d there is one dominant absorption maximum at 410 nm (absorbance 0.50) characteristic of the colloidal AgNPs. The fwhm of colloidal AgNP-PA-3K is 85 ± 2. This indicates that the sample was relatively polydispersed.43 For curve e, the absorption maximum is at 393 nm (absorbance 0.07) with an additional shoulder at 485 nm. There is no evidence of the presence of Ag+ ions in solution from the spectrum (curve e). However, the fwhm of colloidal AgNP-PA-15K is 130 ± 3. This is indicative of the fact the AgNP-PA-15K (curve e) is relatively polydispersed.44 The digital pictures of all the solutions were presented in the inset of Figure 2A. UV−vis spectra shown in Figure 3 demonstrate the stability of colloidal nanoparticles in aqueous solutions. Colloidal AgNPs have inherent tendency to oxidize in the presence of oxygen making them less attractive from the point of view of stability and shelf life.45 The stability of colloidal AgNPs were monitored for a month. In the presence of the PG polymers AgNPs were moderately stable compared to PA-3K and PA15K. In Figure 3A, the UV−vis spectra of AgNP-PG-5K (curve a), AgNP-PG-25K (curve b), AgNP-PG-5K-1m (curve c, sample a after 1 month), and AgNP-PG-25K-1m (curve d, sample in curve b after 1 month) are shown. Absorption maximum of AgNP-PG-5K (curve a) at 399 nm (absorbance 0.54) changes to 398 nm (absorbance 0.48) (curve c) after a month. In addition, the fwhm of colloidal AgNP-PG-5K changes from 65 ± 1 to 61 ± 1 nm after a month. Absorption maximum of AgNP-PG-25K (curve b) at 400 nm (absorbance 0.46) changes to 398 nm (absorbance 0.39) (curve d) after a month. In addition, the fwhm of colloidal AgNP-PG-25K changes from 80 ± 2 to 75 ± 1 nm after a month. The decrease in absorbance is about 0.06 units, and the shift in absorption position is not more than 2 nm in both cases. These slight changes indicate that some of the surface silver atoms are oxidized over the course of 1 month,46,47 but there is minimal size decrease and the AgNPs essentially remain relatively monodispersed. In Figure 3B, the UV−vis spectra of AgNP-PA3K (curve a), AgNP-PA-15K (curve b), AgNP-PA-3K-1m (curve c, sample a after 1 month), and AgNP-PA-15K-1m (curve d, sample in curve b after 1 month) are shown. Absorption maximum of AgNP-PA-3K (curve a) at 410 nm (absorbance 0.50) changes to 400 nm (absorbance 0.36) (curve 4228

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Figure 5. (A, B) FTIR spectra of (i) solid PG-5K (black line), (ii) AgNP-PG-5K conjugates (red line), (iii) solid PG-25 (green line), and (iv) AgNPPG-25K (blue line) conjugates. (C, D) FTIR spectra of (i) solid PA-3K (black line), (ii) AgNg-PA-3K conjugates (red line), (iii) solid PA-15 (green line), and (iv) AgNP-PA-15K (blue line) conjugates.

intensity), and 30 nm (75% intensity) with a PDI of 0.175. The TEM image in Figure 4B (ii) supports these distribution. In Figure 4A two populations were found for AgNP-PA-3K (green line) in 30 nm (at 88% intensity) and 158 nm (at 12% intensity) with a PDI of 0.339. Similarly, two populations were found for AgNP-PA-15K (blue line) in 35 nm (at 68% intensity) and 2.0 nm (at 32% intensity) with a PDI of 0.090. The TEM images in Figure 4B (iii) and (iv) support these distributions. The TEM images of 1 month old samples are given in Figure S2 (Supporting Information). UV−vis spectroscopy is widely used to characterize silver nanoparticles. The surface plasmon resonance (SPR) band corresponds to the AgNPs size, and the red-shift in the SPR band qualitatively indicates an increase in AgNPs size.50 The absorption maxima of 399 nm for AgNP-PG-5K (Figure 2A) indicates the presence of roughly spherical AgNPs, and the corresponding DLS signal (Figure 4A) displays a size of 18 ± 1 nm (PDI 0.090). Previous literature reports indicate that an SPR band of 402 nm corresponds to a diameter of ∼21 nm AgNPs.50 Thus, UV−vis absorption maximum of AgNP-PG-5K provide qualitative size distributions. However, the TEM image for the dried samples show agglomerates of small grains and some dispersed nanoparticles (Figure 4B (i)). In contrast, the other three sets of AgNPs reported here have two size distributions with wide polydispersity as indicated by DLS and therefore do not follow a clear trend corresponding to the SPR absorption maxima. 3.3. Characterization of AgNPs by FTIR Spectroscopy. The changes in specific stretching and bending vibrations of molecules due to conjugation with nanoparticles can be observed by FTIR spectroscopy.51,52 The FTIR spectra of polymer used in this study comprise of several bands as shown in Figure 5, and their assignment are presented in Table 1. The characteristic signals responsible for the polymer−AgNPs

Table 1. Vibrational Band Assignment in FTIR Spectra of Polymer and Polymer Bound AgNPs sample

νC−N (alkyl, cm−1)

PG-5K AgNP-PG-5K

1177 1198

PG-25K AgNP-PG-25K PA-3K

νN−H (cm−1)

νC−N (cyclic, cm−1)

νCO (cm−1)

1273 1319

1665 1675

1180 1198 1129

3189, 3364 3216, 3359, 3437 3183, 3357 3340 3448

1275 1308 1273

AgNP-PA-3K

1147

3400

1317

PA-15K

1125

3367

1273

AgNP-PA-15K

1143

3333

1308

1665 1658 1675, 1698 1667, 1704 1673, 1700 1657

interaction are C−N stretching (alkyl), N−H stretching, C− N stretching (aryl), and CO stretching. The band at 1177 cm−1 in PG-5K is due to C−N stretching vibration of guanidino group and this band shift to 1198 cm−1 in AgNP-PG-5K (Figure 5B). Only two N−H stretching vibrations of guanidino group appear at 3189 and 3364 cm−1 in PG-5K. In the corresponding AgNP-PG-5K conjugates there are three distinct N−H signals are seen at 3216, 3359, and 3437 cm−1 (Figure 5A). The band at 1273 cm−1 in PG-5K is due to C−N stretching vibration of cyclic amido group and this band shift to 1319 cm−1 in AgNP-PG-5K (Figure 5B). The CO stretching vibrations of cyclic amido group appears at 1665 cm−1 in PG5K. In the corresponding AgNP-PG-5K conjugates this signals appears at 1675 cm−1 (Figure 5B). The observed shifts, especially in guanidino C−N and N−H stretches, are indicative of the fact that PG-5K binds to the AgNPs surface facing the guanidino group.51,52 For PG-25K the signals are C−N stretching vibration at 1180 cm−1 (Figure 5B), N−H stretching 4229

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Figure 6. UV−vis spectra of 0.12 mM 4-nitrophenol (4-NP) (black line) and 0.12 mM 4-nitrophenol with 12 mM NaBH4 (red line) (a); 0.12 mM 4-nitrophenol with 12 mM NaBH4 without addition of any catalyst (b); 0.12 mM 4-nitrophenol with 12 mM NaBH4 in the presence of AgNP-PG5K as catalyst (c) and plot of ln(At/A0) against the reaction time for pseudo-first-order reduction kinetics of 4-nitrophenol in the presence of excess NaBH4 (12 mM) in aqueous solutions (d).

vibrations at 3183 and 3357 cm−1 (Figure 5A), C−N stretching vibration at 1275 cm−1 (Figure 5B), and CO stretching vibration at 1665 cm−1 (Figure 5B). In AgNP-PG-25 the corresponding signals are C−N stretching vibration at 1198 cm−1 (Figure 5B), N−H stretching vibrations at 3340 cm−1 (Figure 5A), C−N stretching vibration at 1308 cm−1 (Figure 5B), and CO stretching vibration at 1658 cm−1 (Figure 5B). These change in the FTIR signals of PG-25 while bound to the AgNPs account for the interaction of the polymer with AgNPs.51,52 Similarly, the band at 1129 cm−1 in PA-3K is due to the C−N stretching vibration of the amino group and this band shifts to 1147 cm−1 in AgNP-PA-3K (Figure 5D). Only one broad N−H stretching vibration of amino group appears at 3448 cm−1 in PA-3K. In the corresponding AgNP-PA-3K conjugates there is a distinct N−H signal at 3400 cm−1 (Figure 5C). The band at 1273 cm−1 in PA-3K is due to the C−N stretching vibration of the cyclic amido group and this band shifts to 1317 cm−1 in AgNP-PA-3K (Figure 5D). The CO stretching vibrations of the cyclic amido group appears at 1675 and 1698 cm−1 in PA3K. In the corresponding AgNP-PA-3K conjugates these signals appear at 1667 and 1704 cm−1 (Figure 5D). The observed shifts, especially in the amido C−N and N−H stretches, is indicative of the fact that PA-3K binds to the AgNPs surface facing the amido group.51,52 For PA-15K the signals are the C− N stretching vibration at 1125 cm−1 (Figure 5D), N−H stretching vibrations at 3367 cm−1 (Figure 5C), C−N stretching vibration at 1273 cm−1 (Figure 5D), and CO stretching vibration at 1673 and 1700 cm−1 (Figure 5D). In AgNP-PA-15 the corresponding signals are the C−N stretching vibration at 1143 cm−1 (Figure 5D), N−H stretching vibrations at 3333 cm−1 (Figure 5C), C−N stretching vibration at 1308 cm−1 (Figure 5D), and CO stretching vibration at 1657 cm−1 (Figure 5B). These changes in the FTIR signals of PA-15 are due to the complexation with the nanoparticles’ surface.53

3.4. Application of AgNP-PG-5K for Catalytic Reduction of 4-Nitrophenol. A potential application of metal nanoparticles is the catalysis of certain reactions that would otherwise not occur. In this report we have chosen the reduction of 4-nitrophenol to 4-aminophenol as a model system40,41 in order to evaluate the catalytic activity of AgNPPG-5K. The reduction of 4-nitrophenol (E0(4‑NP/4‑AP) = −0.76 V) by sodium borohydride (NaBH4) (E0(H3BO3/BH4−) = −1.33 V) is thermodynamically feasible but kinetically restricted in the absence of a catalyst. Addition of NaBH4 to a 4-nitrophenol solution changes the light yellow color of the solution to intense yellow due to formation of the 4-nitrophenolate ion.40 This is due to the fact that addition of NaBH4 changes the pH from acidic to highly basic. Accordingly, the absorption peak shifts from 317 to 400 nm (Figure 6a). The catalytic process of this reaction was monitored by UV−vis spectroscopy, as illustrated in Figure 6. Absorption intensity at 400 nm for the 4nitrophenolate ion remained unaltered in the presence of only NaBH4 even after 40 min (2400 s) as shown in Figure 6b. This result indicated that the reduction does not proceed without a catalyst. Figure 6c illustrates the reduction reaction of 4nitrophenol, observed at different time intervals using AgNPPG-5K as the catalyst. According to our DLS and TEM data (Figure 4) AgNP-PG-5K is relatively monodispersed, and thus we have chosen AgNP-PG-5K to explore catalytic activity. Silver nanoparticles though also absorb around 400 nm. In order to avoid the overlap between the SPR band of AgNP-PG5K and 4-nitrophenol, only 25 μL of AgNP-PG-5K was used in a total of 1.0 mL volume to monitor the catalytic action. Absorbance of 25 μL of AgNP-PG-5K is only 0.04 as illustrated in Figure S1 (Supporting Information). In the presence of AgNP-PG-5K and NaBH4 the 4-nitrophenol was reduced, and the intensity of the absorption peak at 400 nm gradually decreased with time and after ∼10 min it fully disappeared (Figure 6c). In the meantime, a new absorption peak appeared 4230

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well monodispersed in solution and their relatively smaller size (∼18 nm). The smaller size consists of a high surface-tovolume ratio and more exposed Ag atoms on the surface, and these exposed atoms potently act as the catalytic sites.55 These results suggested that AgNP-PG-5K might possess an important application in the field of heterogeneous catalysis. 3.5. Mechanism of Reduction of 4-Nitrophenol to 4Aminophenol by NaBH4. Recently, the reaction of sodium borohydride with metal surfaces and its decomposition in presence of metal nanoparticles is an interesting area of research due to its potential application in fuel cells.59,60 The mechanism of reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in the presence of silver nanoparticles is discussed in terms of the Langmuir−Hinshelwood (LH) model59,61 illustrated in Figure 7. Borohydride ions adsorb on the surface of the AgNP nanoparticles and transfer a surface-hydrogen species to the surface of the nanoparticles.59,62 This reversible step can be modeled in terms of a Langmuir isotherm.59 Concomitantly, 4-nitrophenol molecules are adsorbed on the surface of the nanoparticles. This reversible process can also be modeled by a Langmuir isotherm.59 Moreover, the adsorption/ desorption equilibriums and diffusion of reactants to the nanoparticles are considered to be fast.59 The reduction of 4nitrophenol, which is the rate-determining step, occurs due to the reaction of adsorbed 4-nitrophenol with the nanoparticles surface-bound hydrogen atoms (Figure 7).59 When product, 4aminophenol, desorbs leaving free metal surface, the catalytic cycle can begin again.59

at 298 nm and progressively increased in intensity (Figure 6c). This new peak is attributed to the typical absorption of 4aminophenol. The UV−vis spectra in Figure 6c showed two isosbestic points at 280 and 314 nm. This result suggests that the catalytic reduction of 4-nitrophenol exclusively yielded 4aminophenol, without any other side products.54 In the reduction process, the overall concentration of NaBH4 was 12 mM and 4-nitrophenol was 0.12 mM. Considering the much higher concentration of NaBH4 compared to that of 4nitrophenol, the pseudo-first-order kinetics could be applied with respect to 4-nitrophenol in order to determine the catalytic activity of AgNP-PG-5K. The absorbance of 4nitrophenol is proportional to its concentration in solution; the absorbance at time t (At) and time t = 0 (A0) are equivalent to the concentration at time t (Ct) and time t = 0 (C0). The rate constant (k) was determined from the linear plot of ln(At/A0) versus reduction time in seconds, and the constant was estimated to be 5.50 × 10−3 s−1 (Figure 6d). In order to compare the catalytic activity of current catalyst with the ones reported in the literature with AgNP, it is not entirely appropriate to compare the rate constant. Hence, the activity parameter κ = k/mwhich is the ratio of rate constant k to the total mass of the catalyst addedwas used to estimate the catalytic performance of the catalyst.54,55 The reported activity parameters reported by Rashid et al.56 are 1.30, 0.41, and 0.09 s−1 g−1 for coral-like dendrite, banana leave-like dendrite, and spherical Ag nanostructures. Tang and co-workers3 designed AgNP/C nanocomposites having an activity parameter of 1.69 s−1 g−1. Core−shell nanocomposite, Fe3O4-@SiO2-Ag reported by Chi et al.55 and Shin et al.57 have an activity parameter of 7.67 and 275 s−1 g−1, respectively. Very recently, An and coworkers reported microspheres of Fe3O4-@C@Ag with an activity parameter 372 s−1 g−1. All these activity parameters containing nanostructured silver are lower than the 1375 s−1 g−1 of AgNP-PG-5K nanoparticles synthesized in this work. However, this value is comparable to the core−shell microspheres, Fe3O4-@SiO2-Ag-Au, containing additional gold layer as reported by An and co-workers.54 All these parameters are summarized in Table 2. It is worthwhile to mention at this

4. CONCLUSIONS Four different colloidal silver nanoparticles, namely AgNP-PG5K, AgNP-PG-25K, AgNP-PA-3K, and AgNP-PA-15K, were prepared in situ by reducing the Ag+ ions in solutions of polyamino and polyguanidino oxonorbornenes with sodium borohydride (NaBH4). Very recently, Lin and co-workers reported63 a AgNP−nanosilicate platelate hybrid, and these were further blended with water-soluble poly(ether)urethane (PEU). These conjugates promoted antimicrobial activity as well as reduce the inflammatory response.63 Santos et al.64 introduced new AgNP conjugates stabilized by linear polyethylenimine (LPEI). They systematically modified the LPEI framework to create new ligands of polymer stabilizers. They also tested the catalytic activity in the reduction reaction of 4-nitrophenol, and their kinetics were fully analyzed.64 In another very recent study by again Lin and co-workers,65 they emphasized the importance of the existence of polymeric coatings on both AgNPs surfaces and the surfaces they interact with in stabilizing nanoparticles against deposition. This study suggested that although polymeric coatings are stabilizers of AgNPs, they might stabilize AgNPs against deposition unless the collector surface is also polymer coated.65 Polymers reported in this work stabilize AgNPs and kept them in aqueous solutions for more than a month. To our knowledge, this is the first report of stabilizing colloidal silver nanoparticles by cationic polynorbornenes. Compared with the amino polymers (PA-3K and PA-15K), the guanidino analogues (PG-5K and PG-25) better stabilized the silver colloids as evident from the spectroscopic data. Also, there seems to be a polymer size dependence on stability with the shorter PG (PG5K) being more effective than the longer PG (PG-25K). The catalytic activity of AgNP-PG-5K nanoparticles thus prepared was investigated in the reduction reaction of 4-nitrophenol to 4aminophenol by NaBH4. A comparative study revealed that the

Table 2. Summary of the Activity Parameter, κ, Dependent on the Rate Constants of the Reaction (k) and the Amounts of Catalyst Used nanoparticles TAC-Ag-1.0 TAC-Ag-1.4 TSC-Ag-1.4 Ag-NP/C composite Fe3O4-@SiO2-Ag Fe3O4/SiO2-Ag Fe3O4-@C@Ag Fe3O4-@C@Ag-Au AgNP-PG-5K

catalyst (mg) 4.0 4.0 4.0 1.0 1.0 0.02 0.01 0.01 0.004

κ (s−1 g−1)

k (s−1) 5.19 1.65 3.64 1.69 7.67 5.50 3.72 15.80 5.50

× × × × × × × × ×

−3

10 10−3 10−4 10−3 10−3 10−3 10−3 10−3 10−3

1.30 0.41 0.09 1.69 7.67 275 372 1580 1375

ref 56 56 56 3 55 57 54 54 this work

context that gold nanoparticles have also served as catalysts for the reduction of 4-nitrophenol. A recent study by Zhang et al.58 reported the activity parameter, κ, for AuNP-based catalysis and summarized several other AuNP−polymer-based catalysts.58 All of these have a lower κ value than the AgNP-PG-5K conjugates reported here. The good catalytic activity of AgNP-PG-5K nanoparticles is attributed to the fact that these particles are 4231

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Figure 7. Langmuir−Hinshelwood (LH) model for the mechanism of the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4 in the presence of metallic nanoparticles. The AgNP-PG-5K nanoparticles react with borohydride (BH4−) ions to form the silver hydride. At the same time 4-nitrophenol adsorbs on to the silver surface and adsorption/desorption of both the reagents on the surface is fast following Langmuir isotherm. The rate-determining step is the reduction of the adsorbed 4-nitrophenol to 4-aminophenol, and the later desorbs right after formation.

activity parameters, κ, of the nanoparticles synthesized in this work are better than nanocomposites and microspheres with embedded AgNPs reported in the literature. These nanoparticles could be very useful in catalytic applications. Furthermore, the polymers PG and PA being biocompatible, polymer-bound silver nanoparticles could be integrated into various biomedical and biosensor applications and worth studying in the future.

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ASSOCIATED CONTENT

* Supporting Information S

Additional UV−vis spectral data, TEM images of 1 month old silver nanoparticles, and table that summarizes the activity parameter κ (s−1 g−1) of gold nanoparticles from the literature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +1 678 797 2654, +1 678 797 2653; fax + 1 770 423 6744; e-mail [email protected] (B.B.), [email protected] (G.J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by start-up funds from the Chemistry and Biochemistry, Kennesaw State University, and KSU CSM Mentor Protégé funds (BARUAH-05-FY2011-02 and GGABRI-08-FY2010-11). B.B. and G.G. also thank Dr. Mark Mitchell, Department Chair, and Dr. Ronald Matson, Dean of the College of Science and Mathematics, for their enormous support. We acknowledge the Robert P. Apkarian Integrated Electron Microscopy Core of Emory University for use of its Electron Microscopy facility. B.B. and G.G. are thankful to Dr. David Gottfried at Georgia Institute of Technology, NRC, for dynamic light scattering experiments.

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dx.doi.org/10.1021/la305068p | Langmuir 2013, 29, 4225−4234